The spa waters of Kusatsu flow down from Mt. Kusatsu-Shirane, an active volcano 2,160 meters high. Even today, the volcano continues to vent volcanic gas. The crater lake is one of the most acid lakes in the world, with a pH of about 1.0. The surface of the water is a cloudy greenish-yellow. No one is usually permitted to approach within 500 meters of the lake, although there is an observation platform at the edge of the crater where hikers can enjoy a good view.
Yubatake, a springhead located in the center of the town, is the most famous symbol of Kusatsu. The extremely hot water flowing from the spring is cooled as it runs through rows of wooden boxes 40 meters long. The sediment that accumulates in the boxes, known as Yunohana, is collected and sold as a powder which people can use to experience the Kusatsu spa in their bath at home. However, the production volume is limited and stock is always in short supply.
For centuries, people have visited Kusatsu to stay for extended periods in the hope that the spa waters will cure various illnesses. Although now fewer in number, there are still several inns, known as Toji-yado, that hosts long-term visitors. Guests at the inns can bathe not only at the inns, but also at public spa facilities in 18 locations around Kusatsu.